1. Field of the Invention
The present invention relates to the manufacture a semiconductor device, and more particularly, to a polishing pad having a sealing barrier that prevents the infiltration of fluid or moisture onto a transparent window in a chemical mechanical polishing (CMP) apparatus, so as to allow a user to detect the condition of a product during the course of the CMP.
2. Description of the Related Art
Generally, a chemical mechanical polishing CMP apparatus is used in the manufacture of integrated circuits on a semiconductor wafer. The CMP apparatus is mainly used for planarizing or patterning various material layers on wafers. The CMP is employed for polishing of a surface of the wafer with a polishing pad, typically using a polishing fluid, typically a slurry of chemical polishing materials, applied onto a polishing surface.
FIG. 1 is a cross-sectional view of a conventional CMP apparatus, and FIG. 2 is a cross-sectional view of the conventional CMP having a window which serves as a pathway for an optical beam. Referring to FIG. 1, the conventional CMP apparatus for planarizing a semiconductor wafer 10 comprises a platen 35 which revolves around a rotational axis 36 for turning a polishing pad 20 and a head 31 which in turn holds and revolves the wafer 10 around another rotational axis 32. A polishing pad 21 adheres to the upper face of a platen 35. The polishing pad 20 comprises the upper polishing pad 21, which is a hard pad that has a polishing surface contacting the wafer 10, and a bottom pad 23, which is a soft pad located on a back side of the upper polishing pad 21.
When polishing the wafer 10 with the conventional CMP apparatus, it is essential to check the surface state of the wafer 10 to control the polishing process or to detect accurately a polishing end point. Various methods are employed in this respect, and among them, an optical method as illustrated in FIG. 1 is widely adopted. In the optical method, an optical beam 34 is irradiated onto the surface of the wafer 10 by a laser interferometer 32, and the state of the surface of the wafer is analyzed via a reflected beam corresponding with the optical beam 34. For this purpose, an aperture 30 for the passage of the optical beam 34 is provided through the polishing pad 20. This aperture 30 is required to be covered with a transparent window 40 because the polishing slurry or the like can leak into the aperture during the CMP process.
Referring to FIG. 2, since the aperture 30 is covered by the transparent window 40, a fluid derived from the slurry while polishing the wafer 10, or deionized (“DI”) water used for washing, is primarily prevented from leaking into the aperture 30. However, moisture 60 or dew that accumulates on the surface of the window 40 scatters the optical beam. This results in incorrect measurements of the surface state of the wafer 10. Accordingly, the polishing pad 20 must be frequently replaced by a new polishing pad for accurately checking the surface state of the wafer 10 or for detecting the polishing end point.
In order to overcome this problem, several methods to prevent leakage of fluid on the window 40 have been proposed such as in U.S. Pat. No. 6,358,130. However, the moisture 60 still occurs on the surface of the window 40 due to moisture permeation through the bottom pad 23 of the polishing pad 20 as depicted in FIG. 2.
The bottom pad 23 of the polishing pad 20 performs as a buffer layer to improve the uniformity of the polishing. Therefore, the bottom pad 23 is preferably made of a soft material, for example, sponge, which has a flexible texture. The bottom pad 23 is attached to upper polishing pad 21 via a binding layer 51. The lower face of the bottom pad 23 is attached to platen 35 via a binding layer 55. This structure of polishing pad 20 is similar to the IC 1000 pad and IC 1010 pad of Rodel, U.S.A. which are widely used for a CMP apparatus presently.
However, even though this structure of the polishing pad 20 prevents fluid from leaking through the gap between the window 40 and the upper pad 21, preventing fluid from leaking through the bottom pad 23 is practically impossible since the bottom pad 23 is made of a soft material exposed to the slurry or to the DI water. As the polishing process is repeated, fluid leakage becomes more severe, and moisture eventually accumulates on the back side of the window 40 at the aperture 30. For example, moisture will likely occurs on the back face of the window 40 after polishing approximately 2000 times using the IC 1010 pad for a CMP apparatus. Accordingly, the polishing pad 20 will be eventually replaced because of detecting the end point of polishing becomes impossible.